Digital velocity servo

ABSTRACT

A servo circuit for electronically controlling the operation of an electric motor is disclosed. The rotational frequency of the motor is converted into a varying frequency waveform by a tachometer. The tachometer output is applied to a conversion circuit which includes a peak detector that converts the output waveform of the tachometer into a square-wave signal. The square-wave signal is, in turn, connected to one input of an exclusive-OR gate. The peak detector square-wave signal is also applied to a shift register which, after a time delay, applies the same signal to another input of the exclusive-OR gate. With these connections, the exclusive-OR gate produces a square-wave output whose duty cycle is proportional to the speed of the DC motor. This output can then be either amplified and applied directly to the electric motor to drive it, or integrated and applied to an amplifier to drive the motor in accordance with standard servo techniques.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electronic servo circuits for controllingelectric motors.

2. Description of the Prior Art

Present-day electric apparatus often utilizes direct current motors torotate or move mechanical parts. The rotational speed and power outputof such motors are generally controlled by means of servo circuitrywhich controls the electric power applied to the motor. The servocircuitry receives as inputs various measured parameters, such asposition of the motor shaft or the torque produced by the motor,processes these inputs, and produces an output signal to control themotor.

Often, it is desirable to maintain a constant motor speed by means of aservo circuit. In such a case, the motor shaft is generally connected toa tachometer which produces a varying output, often a train of pulses,with a frequency related to the rotational speed of the motor. Afrequency-to-voltage converter is then used to convert the outputdeveloped by the tachometer into a DC voltage which then can beprocessed by standard amplifier circuitry into the power outputnecessary to control the motor.

Many prior art circuits have been designed to perform the necessaryfrequency-to-voltage conversion. In one such system, each output pulseproduced by the tachometer is applied to a monostable multivibratorwhich, after a predetermined time interval, sets a latch. The latch isreset by the next tachometer pulse. The latch circuit produces a squarewave output whose duty cycle is proportional to the error between theactual motor speed and the desired motor speed. The output of the latchis connected in a feedback circuit so that the output duty cycle and,thus, the error is minimized during circuit operation.

Another prior art circuit utilizes phase comparison to control motorspeed. In this type of circuitry the output pulses produced by thetachometer are applied to one input of a phase comparator. A referenceoscillator is connected to the other input of the phase comparator. Thephase comparator produces an output which is proportional to thedifference in phase between the tachometer output and the referencesignal; this output is amplified and used to drive the motor.

Although the above prior art circuits perform the required controlfunction, each has its own problems. The multivibrator circuit describedabove operates to adjust motor speed to a constant determined by thetime constant of the multivibrator. This time constant is in turndetermined by values of electronic components which are subject tochange by thermal effects and aging. Thus to insure constant speed thecomponents used in the multivibrator must be (1) precision components,(2) temperature compensated, or (3) adjusted at the time of manufacture;each alternative is expensive. A second problem with the multivibratorcircuit is that it needs additional circuitry to start the motor from apower-off condition, since the servo loop may not be self-starting ormay be slow in starting.

The phase comparison circuitry requires a phase comparator circuit whichis generally complicated and expensive.

SUMMARY OF THE INVENTION

The foregoing problems and others have been solved in one illustrativeembodiment of a motor servo control circuit in whichfrequently-to-voltage conversion is performed by using a shift registerdelay circuit. Basically, a tachometer waveform having a periodproportional to the rotational period of the motor is first limited toproduce a square-wave output by a peak detector. This output waveform isthen delayed by means of a standard shift register shifted by a precisefrequency reference. The output of the shift register and the output ofthe peak detector are applied to an exclusive-OR gate which produces apulsed output where the pulses have a duration equal to the delayintroduced by the shift register and a pulse spacing equal to thedifference between one-half the period of the peak detector square waveoutput and the delay time. In effect, the pulses produced by theexclusive-OR gate have a duty cycle that varies proportionally to thefrequency of the signal produced by the tachometer. If, for example, thedelay introduced by the shift register is chosen to be one-quarter ofthe period of the tachometer signal at the desired motor rotationalspeed, the duty cycle of the output produced by the exclusive-OR gatewill be fifty percent when the motor is running at the desired speed.The duty cycle will be less than fifty percent if the motor is runningat a speed below the desired speed and will be greater than fiftypercent if the motor is running at a speed higher than the desiredspeed. The output of the exclusive-OR gate may be amplified and applieddirectly to the motor or simply integrated by using a low-pass filterand applied to an amplifier to drive the motor.

The delay introduced by the shift register and the corresponding motorspeed is controlled by a reference frequency that can be easily derivedfrom the system clock which is generally a crystal-controlled clock. Thefrequency-to-voltage conversion circuitry itself needs no temperaturecompensation or adjustment and is, therefore, simple and uncomplicated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the illustrative servo circuit.

FIG. 2 shows a plurality of voltage waveforms at selected points in FIG.1.

DETAILED DESCRIPTION

Referring to FIG. 1, a direct current motor 1.0..0. is controlled byservo circuitry consisting of peak detector 125, shift register 14.0.,gate 15.0., filter 16.0., amplifier 18.0. and driver 185 operating undercontrol of motor control circuitry 195 to run at various constantspeeds. Motor 1.0..0. may illustratively be any conventional type ofdirect current motor, for example, a permanent magnet or field coilexcited motor. As will be hereinafter described, the servo circuitryreceives commands from motor control 195 causing the circuitry tocontrol the electric power applied to motor 1.0..0..

The servo circuitry monitors the rotational speed of motor 1.0..0. bymeans of a tachometer connected to the motor shaft. Specifically, thetachometer comprises slotted wheel 115 and variable reluctance coilpickup 12.0.. The tachometer arrangement is coupled directly to motor1.0..0. by means of shaft 11.0..

The tachometer arrangement used to sense the rotational speed of motor1.0..0. may be any conventional arrangement, including a photocell andlight and slotted wheel arrangement or the slotted disk and magneticpickup arrangement shown in FIG. 1. Well-known detectors of this typetypically produce a sinusoidal-like output as shown in FIG. 2A. Eachpeak in the waveform corresponds to the passage of a slot in wheel 15past pickup 12.0. or, in the case of an optical arrangement, the passageof a slot by the photocell and light arrangement.

The varying voltage produced by the tachometer device is applied to peakdetector 125 which serves to convert the analog waveform into a squaredwaveform suitable for use with digital electronics. Peak detector 125may be of any conventional design which produces a square wave outputhaving a signal transition corresponding to each positive or negativepeak in the input waveform. For example, typical prior art circuitrywhich may be used to perform this function consists of a differentiatorand a zero crossing detector. Another device which performs a similarfunction is a subtraction peak detector in which the input waveform issubtracted from a slightly delayed copy. The difference becomes zero atthe peaks of the input waveform. Due to the delay introduced in theprocessing of the signal waveform with this type of subtractioncircuitry, output signal transitions corresponding to input signal peaksare delayed by a small amount of time.

Assuming that peak detector 125 is a subtraction peak detector, theoutput waveform produced by the tachometer output is shown in FIG. 2B,which output consists of a square waveform with period T having a signaltransition corresponding to the positive and negative peaks in thetachometer output.

The peak detector output is then applied to the frequency-to-voltageconverter circuitry consisting of shift register 14.0. and exclusive-ORgate 15.0. which produces a square-wave output having a duty cycleproportional to the frequency of the input waveform, which output isthen used to control the electric power applied to motor 1.0..0..

Specifically, the output of peak detector 125 is applied via leads 13.0.and 135 to upper input 145 of exclusive-OR gate 15.0. and shift register14.0., respectively. The output of peak detector 125 is also applied tomotor control 195 for the purpose of slowing down the motor as will belater described.

Shift register 14.0. is of conventional design and consists of aplurality of stages connected in series. Data applied to the input isshifted from stage to stage under control of a clocking signal appliedto shift input 165. On the rising edge of a clock pulse applied to shiftinput 165, data present at the shift register input is sampled and onthe falling input of a clock pulse applied to shift register 14.0., thesample data is shifted into the first stage. Each stage then shifts thedata stored therein to the next sequential stage and the last stage dataappears on the output lead 146. Thus, shift register 14.0. produces asignal equivalent to that present at its input after a delay in time.The amount of delay is determined by the number of stages in shiftregister 14.0. multiplied by the time interval between clock pulsesapplied to shift input 165. To insure proper operation of the circuitry,shift register 14.0. must have a plurality of stages. The reason forthis is that the signal transitions in the output of peak detector 125occur asynchronously with respect to the clock pulses applied to shiftinput 165. Therefore a sampling delay of up to one clock pulse mightoccur before the input to register 14.0. is sampled by the internalcircuitry. This unpredictable sampling delay causes a variation in theactual delay introduced by register 14.0. into the servo circuitry. Asthe number of shift register stages is increased the sampling delaybecomes a smaller percentage of the actual delay and thus the errorintroduced by it decreases. Illustratively, shift register 14.0. mayconsist of 18 stages.

Shift input 165 receives clocking pulses from programmable divider 17.0.which, in turn, receives a constant frequency clock pulse on its inputlead 175 from the system clock (not shown). Advantageously, the systemclock may be crystal-controlled and is therefore extremely stable overtime despite temperature variations and aging of the other components inthe circuitry. Divider 17.0. is also controlled by signals on lead 191from motor control 195. By appropriate signals, control 195 (which mightillustratively include a microprocessor) may control the divisionconstant in divider 17.0. thus effectively controlling the clockfrequency applied to register 14.0.. A change in the frequency of clockpulses applied to register 14.0. directly changes the delay and will, inturn, change the speed of the motor, as will hereinafter be described.

Assume, for the purposes of illustration, that the number of shiftregister stages and the clock pulse frequency is such that the delayintroduced by shift register 14.0. is equal to one quarter of the period(T) of the tachometer waveform at rated velocity. The output of shiftregister 14.0. is then as shown in FIG. 2C.

The output of register 14.0. is applied to the lower input ofexclusive-OR gate 15.0.. Exclusive-OR gate 15.0. is a well-known logicdevice which produces a high signal at its output 155 when either one,but not both, of its inputs 145 and 146 are high. With the inputwaveform 2B applied to input 145 and waveform 2C applied to input 146,the output produced on output lead 155 is shown in FIG. 2D. It consistsof a square wave output with a 5.0. percent duty cycle having anamplitude equivalent to the supply voltage 151 (magnitude V) applied togate 15.0..

As will be described in detail below, the signal produced on the output155 of gate 15.0. has a duty cycle proportional to the frequency of thesignal from peak detector 125. This signal is then filtered andamplified to produce the control signal for motor 1.0..0.. Specifically,output 155 of gate 15.0. is applied to low pass filter 16.0. which mayillustratively be a single pole filter consisting of a resistor andcapacitor. Filter 16.0. integrates the output signal (waveform 2D),producing a D.C. value having a magnitude of approximately V/2.

This D.C. signal is applied to the upper input of feedback amplifier18.0.. The lower input of amplifier 18.0. receives a reference voltageon lead 184 from the junction of resistors 182 and 183. Resistors 182and 183 from a voltage divider between voltage source 181 and ground.Advantageously, voltage source 181 is the same source as source 151 usedto provide power to exclusive-OR gate 15.0.. Variations in the magnitudeV of the source will then be automatically cancelled by the circuitry.

The values of resistors 182 and 183 may be chosen to give any fractionof the source voltage V. The selection of resistors 182 and 183determines the operating point of the system for a predetermined delayintroduced by register 14.0.. For example, if register 14.0. introducesa delay of T/4, the servo system will be balanced if the value ofresistor 182 equals the value of resistor 183. The output of amplifier18.0. is applied to driver circuit 185 which in turn produces a drivesignal on its output 186 to operate motor 1.0..0.. Motor 1.0..0. isthereby provided with the appropriate electric power to cause it to runat a predetermined constant speed.

Assume now that some condition, such as increasing the load on motor1.0..0., causes its speed to decrease. When this happens, the period ofthe pulses produced by peak detector 125 in response to the tachometeroutput increases as shown in FIG. 2E (i.e., the frequency decreases).

Similarly, the period of the signal output of shift register 14.0.increases as shown in FIG. 2F. The delay introduced by shift register14.0., however, remains the same (T/4) since the delay is determined bythe clock pulses applied to shift lead 165 and these clock pulses do notchange frequency for a given operations condition. As shown in FIG. 2G,the duration of the pulses produced at the output 155 of gate 15.0.remains constant, but their frequency decreases. This causes theeffective duty cycle of the signal from the exclusive-OR gate 15.0. todecrease. Accordingly, the magnitude of the D.C. signal produced byfilter 16.0. is lower.

Amplifier 18.0. responds to this lower signal at its negative input byincreasing its output, in turn causing driver 185 to increase its outputto motor 1.0..0.. Motor 1.0..0. is thereby caused to increase its speed.

Assume now instead that external conditions cause the speed of motor1.0..0. to increase. The period of the output signal of peak detector125 decreases (i.e., its frequency increases) as shown in FIG. 2H. Theperiod of the waveform produced at the output of shift register 14.0.also decreases as shown in FIG. 2J. However, as in the previous case,the delay remains the same, so the duration, or width, of the pulsesproduced by gate 15.0. therefore remains the same but the frequencyincreases, causing the effective duty cycle of the signal at the output155 to increase. Responsive to an increased duty cycle at its input,filter 16.0. produces an output with a larger magnitude. Upon receivingthe output with increased amplitude amplifier, 18.0. decreases itsoutput causing driver 185 to decrease its drive to motor 1.0..0.,thereby causing the motor speed to decrease.

Thus, advantageously motor control 195 may control the operation of theservo circuitry to dynamically switch motor 1.0..0. from one speed toanother by changing the division constant of programmable divider 17.0..As explained above, a change in the division constant will effectivelychange the amount of delay introduced into the circuitry by register14.0. and this the pulse width. The amount of delay is directlyproportional to the duration of the output pulses produced by OR gate15.0. and thus directly affects the duty cycle which in turn changes thedrive applied to motor 1.0..0.. A change from one speed to another maybe effected in several ways. For example, in some servo systems powermay be reversed thus slowing the motor. In order to keep the systemstable, the servo loop in this type of system must be able to sense thedirection of rotation of the motor. In other systems, however, such asthe circuit described herein, the servo circuit cannot sense thedirection of rotation of the motor; only the speed error can be sensedby the tachometer. In this type of system, to effect a change from ahigher speed to a lower speed the drive provided to motor 1.0..0. maysimply be reduced and the motor allowed to coast to a lower speed.Advantageously, the servo circuitry may be used to produce positivebraking to slow the motor down.

Positive braking may be accomplished by motor control 195 applying areversing signal to drive circuit 185 by means of lead 19.0.. Thereversing signal introduces an inversion into the driver amplifierscausing them to apply a reverse current to motor 1.0..0.. The servocircuitry, sensing a speed slow-down in motor 1.0..0., causes amplifier18.0. to apply a larger signal to driver 185 in accordance with theprinciples described above. In response to the larger signal, however,driver 185 only applies a larger reverse current to motor 1.0..0.,slowing it even faster.

In the braking configuration the circuitry is in a so-called "positivefeedback" configuration. If the circuitry remained in thisconfiguration, the motor speed would eventually reverse and increase inthe reverse direction indefinitely. To prevent such an occurrence motorcontrol 195 monitors the period of the pulses produced by peak detector125. When this period exceeds a reference time interval, control 195controls driver 185 to remove all power from motor 1.0..0. allowing itto coast to the lower speed.

Obvious variations to the principles of operation of the inventiondescribed above would occur to those skilled in the art. For example,with some types of motors, the output of gate 15.0. may be amplified andused to directly drive motor 1.0..0., thereby eliminating filter 16.0.,amplifier 18.0., and driver 185. This and other similar variations arewithin the scope of the invention.

What is claimed is:
 1. An electric motor servo control system forcontrolling the speed of an electric motor comprising:A. speed signalmeans for producing a speed signal consisting essentially of pulseswhose time period of repitition is proportional to the rotational periodof the motor, B. delay means for receiving the speed signal from saidspeed signal means and producing a delayed speed signal consistingessentially of pulses delayed from the pulses of the speed signal by apredetermined time interval that is less than the duration of the pulsesof the speed signal whenever the speed of the motor is within anintended speed range, C. logical-combination means for receiving saidspeed signal and said delayed speed signal and for generating a sequenceof constant-width, variable-frequency motor-control pulses wherein thepulse width is substantially equal to said predetermined time intervaland the pulse frequency is proportional to the repetition frequency ofthe speed signal, the duty cycle of the motor-control pulses therebyvarying in response to variations of the rotational speed of the motor,and D. means for receiving the motor-control pulses from saidlogical-combination means and adapted for coupling to the motor to drivethe motor in accordance with the duty cycle of said motor-controlpulses.
 2. An electric motor servo control system as recited in claim 1wherein said delay means includes a shift register means having an inputterminal for receiving said speed signal and an output terminal at whichit produces said delayed speed signal, said delay means furtherincluding means for shifting the contents of said shift register fromits input terminal to its output terminal at a predetermined rate.
 3. Anelectric motor servo control system as recited in claim 2 wherein saidlogical-combination means comprises an exclusive-OR gate having inputterminals for receiving said speed signal and said delayed speed signaland an output terminal at which it produces said motor-control pulses.4. A servo system for controlling an electric motor comprising:A. Atachometer connected to said motor for producing a tachometer signalhaving a period proportional to the rotational period of said motor, B.a peak detector responsive to said tachometer signal for producing asquare wave output having signal transitions corresponding to the peaksin said tachometer signal, C. a shift register responsive to said squarewave output for producing an output equivalent to said square waveoutput delayed by a predetermined interval of time, D. an exclusive-ORgate responsive to said square wave output and said shift registeroutput for generating a motor-control signal consisting essentially ofconstant-width, variable-frequency motor-control pulses wherein thepulse width is substantially equal to said predetermined interval oftime and the pulse frequency is proportional to the repetition frequencyof the tachometer signal, and E. means responsive to said motor-controlpulses for controlling the rotational speed of said motor in accordancewith the duty cycle of the motor-control signal.
 5. An electric motorservo control system as recited in claim 4 additionally comprising meansfor braking the motor in response to a braking command, said brakingmeans including:i. means responsive to said braking command forreversing the current supplied to said motor, ii. means connected tosaid motor for monitoring the rotational period of said motor, and iii.means for interrupting said current supplied to said motor when saidrotational period of said motor exceeds a second predetermined timeinterval.
 6. An electric motor servo control system as recited in claim1 wherein said logical-combination means comprises an exclusive-OR gatehaving input terminals receiving said speed signal and said delayedspeed signal and an output terminal at which it prouces saidmotor-control pulses.